This invention relates to diesel engines and, more particularly, to control of cylinder compression ratio using an intake cam phaser.
It is known in the art to provide means for varying the compression ratio of a diesel engine in order to provide a relatively high compression ratio for cold starting and warm-up, where compression ignition is more difficult, and to provide reduced compression ratios for operating in other modes, particularly at high loads and speeds, to reduce peak combustion pressures and temperatures. Recently the emphasis for such arrangements is primarily to minimize emissions of nitrogen oxides (NOx) by operating at lower compression ratios where this is possible. Many devices have been proposed for compression ratio variation, including variable valve timing mechanisms and engine components such as pistons and cylinder heads with movable combustion chamber walls. In general these devices are relatively complex and add significant cost to the manufacture of an engine.
In spark ignition engines, cam phasers are known as simple devices for varying cam timing and thus changing valve timing to the extent permitted by the camshaft layout. These devices normally provide for advancing or retarding the cam timing in order to obtain desirable combustion characteristics. It is believed that cam phasers have not been utilized on diesel engines because the piston to cylinder head clearance is so small that altering intake and exhaust valve timing may result in contact of the pistons with the valves. A simple and relatively low cost apparatus and method for controlling compression ratio in a diesel engine is desired.
The present invention provides a desired engine combination by the addition of a cam phaser capable of retarding the timing of only the intake valves of a diesel engine in order to reduce its compression ratio. A typical diesel engine has cylinders and pistons defining expansible combustion chambers into which air is admitted and compressed during compression strokes of the pistons. Compression increases the air temperature so that injected fuel is self-ignited and burns, creating power to drive a crankshaft. Intake and exhaust valves, actuated by separate crankshaft driven intake and exhaust camshafts, control timed admission of air to and discharge of exhaust products from the combustion chambers.
In accordance with the invention, a cam phaser is mounted on the intake camshaft and is operable to selectively retard timing of only the intake valves relative to the crankshaft. The purpose of retarding timing of the intake valves is to retard valve closing sufficiently to shorten the effective compression strokes of the pistons and thus reduce the effective compression ratio. This occurs when the intake valves remain open past piston bottom dead center for a desired period into the normal compression stroke phase of engine operation. This reduces compression pressures in the combustion chambers so that combustion temperatures are reduced and exhaust emissions, primarily NOx, may be thus limited under conditions of warmed-up engine operation.
Additional reductions in combustion temperatures can be achieved, in conjunction with use of an intake cam phaser in turbocharged or supercharged diesel engines, by increasing the intake boost pressure to maintain constant trapped air mass in the cylinder, even when intake camshaft phase delay is utilized. This approach allows maintaining lower combustion temperatures, thus inhibiting NOx formation by preventing increases in fuel-air ratio as compression ratio is decreased.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.